For outgoing messages, the value for the service_type field is taken from one of three places (in order).
1. The value of the FORCE_SERVICETYPE connector option.
2. The message, option MGP_OPTION_SERVICETYPE (41).
3. The value of the SERVICETYPE connector option.
The default value is an empty string. The valid values are defined by the operator you’re sending the messages to, so they need to tell you what value they want to see here.